The present invention relates to a sheet-like light source device. More particularly, it relates to a sheet-like light source device which is employed as a backlight of a liquid crystal display device, wherein the sheet-like light source device is capable of eliminating irregularities in brightness generated at peripheries of electrode portions of a rod-like light source such as a lamp.
A sheet-like light source device as illustrated in FIGS. 5 and 6 is conventionally used a backlight in a liquid crystal display device. Such a conventional sheet-like light source device is composed of a lamp 21 serving as a rod-like light source, a lamp reflector 22 serving as a tubular reflection member, a light-conducting plate 23, and a reflecting sheet 24 serving as a planar reflecting member. The lamp 21 mainly comprises a cold-cathode tube (CFL), and light emitted from the lamp 21 is made incident on the light-conducting plate 23, either directly or via the lamp reflector 22. A transparent material is used as the light-conducting plate 23, and polymethyl methacrylate (PMMA) exhibiting high transmission rate of light is generally used. As illustrated in FIG. 6, light which is made incident on a light-incident surface 23a of the light-conducting plate 23 at an angle of xcex821 is refracted, and its angle xcex822 within the light-conducting plate 23 will be in a range of 0 to 42xc2x0 owing to Snell""s law of fraction. Light will thereafter hit against a front surface 23b or rear surface 23c of the light-conducting plate 23, wherein the incident angle xcex823 is represented by 90xc2x0-xe2x88x92xcex822 and will be in a range of 48 to 90xc2x0. Since a total reflection angle of PMMA is 42xc2x0, xcex823 will be a total reflection angle. In other words, all of light made incident from the light-incident surface 23a of the light-conducting plate 23 will satisfy a total reflection condition and will not be emitted from the front surface 23b or rear surface 23c of the light-conducting plate 23. The rear surface 23c of the light-conducting plate 23 is thus devised to emit light therefrom to the front surface 23b by forming a scattering portion 25 generally through white printing. Light will be scattered when hitting against this scattering portion 25 to thereby break the total reflection condition of light within the light-conducting plate 23, so that light will be emitted from the front surface 23b which is a display surface side of the light-conducting plate 23. This scattering portion 25 is made of a material which does not perform absorption, but only perform scattering of light. Light which has been scattered at the scattering portion 25 will be also scattered to the rear surface 23c which is a non-display surface side of the light-conducting plate 23, so that a reflecting sheet 24 is placed on the rear surface 23c for reflecting this light to the front surface 23b on the display surface side. The reflecting sheet 24 is generally disposed to extend to the interior of the lamp reflector 22 as illustrated in FIG. 6.
Accompanying downsizing of personal computers in these years, narrow framing (that is, decreasing the width of frame 26 as illustrated in FIG. 7) of liquid crystal display devices is also being strongly wanted. However, the lamp 21 includes not only an essential light-emitting region but also a region which does not emit light such as electrode portion 21a, wherein the latter affects the display portion of the backlight (wherein the electrode portion 21a is illustrated to be drawn outside of the frame 26 for ease of understanding in FIG. 7), so that irregularities in brightness X are generated in proximities of both ends of the lamp 21 at which luminous energy is decreased. If it should be possible to effectively eliminate such irregularities in brightness X, the length of the lamp 21 can be shortened up to a dimension of the display portion. In other words, the outer diametric dimension of the backlight can be reduced which is an extremely important technique for achieving narrow framing.
A conventional method which is taken for eliminating irregularities in brightness X is a method for increasing the scattering portion 25 formed on the rear surface 23c of the light-conducting plate 23 of FIG. 6 through printing. However, light-conducting plates are becoming popular in these years which are of a type with no scattering portion 25 being formed on the rear surface 23c of the light-conducting plate 23 through printing.
As a method for eliminating irregularities in brightness X without printing, there is disclosed a technique wherein the light-incident surface of the light-conducting plate is formed as a coarse surface for increasing the amount of scattered light in approaching both lateral ends of the light-incident surface (see Japanese Unexamined Patent Publication No. 160036/1997); however, employing this technique requires an additional process of forming the coarse surface using sandpapers or the like after injection molding of the light-conducting plate.
The present invention has been made in view of the above problems, and provides a sheet-like light source device capable of effectively eliminating irregularities in brightness X without performing printing and capable of being easily manufactured.
In accordance with the present invention, there is provided a sheet-like light source device of side-light type comprising a light-conducting plate made of a light-transmitting material, at least one rod-like light source disposed proximate to a light-incident surface comprising at least one lateral side end portion of the light-conducting plate, a tubular reflecting member covering portions other than a surface facing the light-conducting plate of the rod-like light source, and a planar reflecting member disposed proximate to a position facing a rear surface of the light-conducting plate,
wherein sloped surfaces which are inclined with respect to the light-incident surface of the light-conducting plate are formed proximate to positions of the light-incident surface of the light-conducting plate at which a luminous energy of the rod-like light source is decreased.
It is preferable that the sloped surface is inclined with respect to the light-incident surface at an angle of not less than 6xc2x0.
It is preferable that the sloped surface increases in width in approaching the positions at which the luminous energy of the rod-like light source is relatively decreased.
It is preferable that angles formed by the sloped surface and the light-incident surface of the light-conducting plate increase in approaching the positions at which the luminous energy of the rod-like light source is relatively decreased.
It is preferable that the positions at which the luminous energy of the rod-like light source is decreased are positions at which the electrode portions on both ends of the rod-like light source are located.
It is preferable that the sloped surface is formed on a front surface side and/or rear surface side of the light-incident surface of the light-conducting plate.